Not far from Yosemite's waterfalls and in the middle of California's redwood forests, Mexican drug gangs are quietly commandeering U.S. public land to grow millions of marijuana plants and using smuggled immigrants to cultivate them.

Pot has been grown on public lands for decades, but Mexican traffickers have taken it to a whole new level: using armed guards and trip wires to safeguard sprawling plots that in some cases contain tens of thousands of plants offering a potential yield of more than 30 tons of pot a year.

"Just like the Mexicans took over the methamphetamine trade, they've gone to mega, monster gardens," said Brent Wood, a supervisor for the California Department of Justice's Bureau of Narcotics Enforcement. He said Mexican traffickers have "supersized" the marijuana trade.

Interviews conducted by The Associated Press with law enforcement officials across the country showed that Mexican gangs are largely responsible for a spike in large-scale marijuana farms over the last several years.

Read Associated Press Story at ABCNews.com